As an electrolyte for batteries, liquid or gel electrolytes have heretofore been used because of their ionic conductivity. An outer casing having high mechanical strength must be used for batteries to avoid liquid leakage leading to damage of instruments. This poses a problem, for example, interferes with miniaturization or weight reduction of batteries.
To solve the above-mentioned problem, solid polymer electrolytes are being examined. Solid polymer electrolytes have good processability and pliability and therefore batteries can have various shapes without restriction. Further, solid polymer electrolytes have no fluidity and hence batteries possess a high safety.
For example, a proposal has been made wherein an ethylene oxide-propylene oxide copolymer having incorporated therein an alkali metal salt is adopted as an ion-conductive solid electrolyte (Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] No. S61-83249, JP-A S63-136407 and JP-A H2-24975). However, higher ionic conductivity and higher mechanical strength are now desired for the ion-conductive solid electrolyte. Further, solid polymer electrolytes used for batteries are handled usually in the form of film in the course of production. Therefore, it is required that solid polymer electrolytes have good film-forming property, and the film thickness is rendered as thin as possible to enhance the capacity of battery.
A polymeric material having high mechanical strength including high tensile modulus is required as a material for solid electrolyte to give a film of solid polymer electrolyte exhibiting high mechanical strength even though the film thickness is thin. However, such polymeric material generally exhibits a high melt viscosity and poor fluidity in a molten state at a film-forming process. To lower the melt viscosity, a plasticizer is usually incorporated in the polymeric material, but, a large amount of plasticizer must be incorporated to lower the melt viscosity to the desired extent. The incorporation of a large amount of plasticizer poses other problems in that the polymeric material tends to stick to a roll or other devices at a film-forming process, especially at an extruding step, with the results that the processability is reduced and the resulting shaped or formed solid polymer electrolyte has poor mechanical strength. Further, the incorporation of a large amount of plasticizer results in reduction of ionic conductivity of the solid polymer electrolyte, leading to deterioration of battery performance.
In the case when a polyether polymer is used as a solid electrolyte, a procedure is generally adopted wherein a polyether polymer having a crosslink-forming reactive functional group is formed into a film, and the film is cured with a radical initiator such as an organic peroxide, or with active radiation whereby a crosslink is formed. The polyether polymer having a crosslink-forming reactive functional group is prepared by copolymerization of a crosslink-forming monomer. Therefore, crosslinking tends to occur to some extent at the step of copolymerization to produce a polymer having a crosslinked structure. When a solid polymer electrolyte film is formed from a polymer containing a large amount of crosslinked structure, the polymer has poor processability and the resulting electrolyte film has poor uniformity with the results that performance and safety of batteries are reduced.
An attempt has been made wherein a polymerization catalyst prepared by reaction of triisobutylaluminum with an organic acid salt of diazabicycloundecene and with phosphoric acid, is used for polymerization to reduce the undesirable crosslinking occurring at a polymerization step (Japanese Examined Patent Publication No. S56-51171). Further, the present inventors have proposed a method of conducting polymerization in the presence of Lewis base substance having no active hydrogen atom to further reduce the undesirable crosslinking occurring at a polymerization step (Japanese Patent Application No. 2001-341155). However, the contents of crosslinked structure in the collected polymers are liable to have poor evenness. Thus, a method of producing a polyether polymer having a more reduced content of crosslinked structure is desired.